Vagal nerve stimulation (VNS) systems and methods are described, for example, in U.S. Pat. Nos. 7,778,703, 7,783,362, 8,401,604, 8,473,068, 8,712,547, U.S. Patent Appl'n. Publ'n. 2013/0184773 and U.S. Patent Appl'n. Publ'n. 2014/0155949, as well as in Robert J. Henning et al., “Vagal nerve stimulation increases right ventricular contraction and relaxation and heart rate,” Cardiovascular Research 32, 1996, 846-853.
VNS is applied in dependence on a parameter indicating autonomic tone of the patient, for example the heart rate (HR). VNS is equivalent to stimulation of the parasympathetic system. Therefore, VNS is not desired when the HR is too low. Common VNS systems are configured in a way that stimulation stops when the HR sinks below a certain value.
U.S. Pat. No. 6,473,644 B1 describes methods for adjusting VNS stimulation frequency in dependence on the ventricular HR of the patient. A damped feedback loop with hysteresis is used, with the HR as the controlling parameter and VNS stimulation frequency as the output.